[PD] Understanding the mechanics of rebuilding Pd's DSP graph

Sun Sep 20 18:53:05 CEST 2015

Thanks, Matt.  It seems like your point #1 is one of the main reasons to use Pd.
As far as best practices-- I think the first thing is to go through and see under what conditions thegraph could be selectively rebuilt.  Once there's a proof of concept that this can be done while still guaranteeing deterministic behavior, the best practices are simply documentation of that feature.
But it's really an all or nothing thing.  The current conservative approach is way less workto debug and maintain than trying to be efficient for dynamic patching.  If we make it possible toselectively rebuild, it's not worth the effort if the user still feels like they can trigger a completerebuild just by sneezing.  (Or if they find that the constraints keep them from building anythinginteresting.)  Practically speaking, I think that means Pd has to be smarter about array resizing,and tracking whether wireless tilde objects navigate outside of a canvas/abstraction/toplevel.
Seems hard, but of course I don't know the tricks of the trade in this domain.

-Jonathan

     On Sunday, September 20, 2015 3:56 AM, Matt Barber <brbrofsvl at gmail.com> wrote:

 Things are a lot more encapsulated in SuperCollider, and it can afford to be more efficient because 1) the DSP flow doesn't need to be in a strict correspondence with what appears in a GUI graph, 2) the user is not responsible for the DSP flow unless they want to be and do it on purpose with head/tail instantiation or node numbering, and 3) there are only a few ways of passing information between ugens: directly as arguments to another ugen, variables, and buses. Data in an SC Synth is more protected from the outside than anything in Pd is (as abstractions in Pd are still in the global space despite the dollarsign locality tricks), and the bus system controls I/O to and from Synths much more strictly. Pd doesn't enforce any of that because of its pledge to keep things global; this makes it extremely flexible, but at the price of a potentially more convoluted DSP graph.
What would be interesting to know is whether there are best practices for patching that would help out the ugen graph routine.
On Sun, Sep 20, 2015 at 12:30 AM, Jonathan Wilkes <jancsika at yahoo.com> wrote:

Matt-- I don't believe that bug has been fixed.
Roman-- I haven't looked closely at the relevant code, but it looks like Pd recalculates the graph-- a single graph for the runninginstance of Pd-- every time you add/remove a tilde object.  (Not sure aboutcontrol objects, but it's easy to test.)
The reason I'm comfortable speculating about this is the existence ofwireless tilde objects like [throw~] and [catch~] which use globalreceiver names.  When you change an object inside a tiny patch with100 other patches open in the same Pd instance, how would Pd know thatyou aren't altering a [throw~] which has a [catch~] in one of the 100 otherpatches?  Same for [send~]/[receive~], [delwrite~]/[delread~]/[vd~],[table]/[tab*~], etc.
Here's the dsp_tick routine in d_ugen.c:

void dsp_tick(void)
{
    if (dsp_chain)
    {
        t_int *ip;
        for (ip = dsp_chain; ip; ) ip = (*(t_perfroutine)(*ip))(ip);
        dsp_phase++;
    }
}

That is-- execute each dsp routine in the global array of dsp routines untilthere are no more dsp routines to execute.
But this makes me wonder-- how does Supercollider "do its thing"?  Seems likeit has an interface to add/remove parts of its dsp graph, and it can do so in a 
much more efficient manner.
-Jonathan

     On Saturday, September 19, 2015 10:56 PM, Matt Barber <brbrofsvl at gmail.com> wrote:

 One more thing to think about is how the DSP graph is handled using dynamic patching. For a long time there was a "bug" where the last audio object added didn't trigger a recalculation and would be left out of the DSP graph until the next edit. Is this still the case? The workaround, if I remember correctly, was to add one last dummy object at the end of dynamic patching.
Matt
On Thu, Sep 17, 2015 at 5:55 PM, Roman Haefeli <reduzent at gmail.com> wrote:

Hi all

First, I'm not even sure if 'DSP graph' is the correct term. Pd's
documentation[1] states that all DSP objects are internally arranged
into a linear order which I believe is often called 'DSP graph'. There
are apparently some actions that cause this DSP graph to be rebuilt.
Rebuilding takes time and is often the cause of audio drop-outs. I would
like to have a better understanding of the mechanics going on behind the
scenes with the hope to be able to optimize my Pd programming.

One thing I'd like to know: Is there one graph for all patches in a
certain instance of Pd? It seems that adding a tilde-object to a patch
causes the DSP graph to be recalculated. Now, if _everything_ is in the
same graph, this would mean the whole graph needs to be recalculated
when adding objects (or abstractions containing tilde-objects, for that
matter), no matter where I put them. It would make no difference whether
I have one big patch with 1000 tilde-objects loaded or 100 smaller
patches with 10 tilde-objects each, when adding new objects, would it?
Is the time it takes to recalculate the graph only dependent on the
number of tilde-objects running in the current instance of Pd? If so, is
that a linear correlation? 10 times more tilde-objects means it takes 10
times as long to recalculate the graph? Or is it even exponential? There
is no way to partition the graph and update only one partition, is
there?

On a related note, I made the following observation and I'm wondering
if/how that is related to the DSP graph: I create a minimalist patch
with a small [table foo 100] and I measure the time it takes to 'resize'
it to 99 with [realtime]. On my box, this takes 0.01 ms. I expected it
to be fast, since memory access is very quick. Now, I additionally load
a much more complex patch with many tilde-objects. I 'resize' the table
again and it still takes only 0.01ms. Now I put a [tabread~ foo]
somewhere in the patch. Now,'resize'-ing the table foo to 100 takes
20ms. Even if I remove the [tabread~ foo] again, resizing the table
still takes at least 20ms. There is no way to make it fast again except
restarting Pd. I also figured out that when only a non-tilde [tabread
foo] is refencing the table I'm resizing, the resizing keeps being fast.
Only when tilde-objects are referencing the table, resizing that very
table becomes slow. The actual time seems dependent on the complexity of
the loaded patch(es). And it also corresponds with the time it takes to
send 'dsp 1' to pd (when dsp is switched off).

Why is resizing tables so much slower, when tilde-objects are
referencing it? I noticed that even resizing very small tables can be a
cause for audio drop-outs. I wonder whether 'live-resizing' should be
avoided altogether.

Yeah, that's a bunch of questions... Even when knowing the answer to
only some them, it might clear things up quite a bit.

Roman

[1] "Pd sorts all the tilde objects into a linear order for
running." ( http://msp.ucsd.edu/Pd_documentation/x2.htm#s4.2 )

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